The growth of thin films is a core process in the production of many electronic and optoelectronic devices. It is also often the most costly and time consuming step and any increase in the efficiency of thin film deposition would almost certainly lead to a reduction of the production costs.
Thin films are deposited onto a substrate material which carries them. Substrates can be made up of virtually any solid material. One of the main criteria for a substrate is that it has to withstand the process conditions during the thin film deposition and subsequent processing, especially with respect to the temperature.
Thin films can be deposited in a variety of ways including chemical vapour deposition (CVD), molecular beam epitaxy (MBE), Vapour Phase Epitaxy (VPE), Atomic Layer Deposition (ALD), sputtering, electro-deposition, thermal evaporation etc. This is by no means an exhaustive list and new techniques for thin film deposition are constantly being developed. For the purpose of this patent, these techniques are split into two categories: directional and conformal techniques. In directional techniques the mean free path of the particles deposited is larger than the distance between the source of the material and the substrate (e.g. sputtering, thermal evaporation, MBE). This means that deposition on some parts of the substrate can be limited, e.g. by putting obstacle into the path between the source and the substrate surface or by structuring surface structure itself (i.e. no film will be deposited onto parts of the surface which are parallel to the path between the source and the substrate.
In conformal techniques the thin film is deposited everywhere on the substrate surface. The mean free path is shorter than the distance between the source and the substrate. In fact, the path a particle takes can often be described by Brownian Motion.
For many techniques the process conditions can be varied so that the deposition is either conformal, directional or somewhere in-between.